Transmission gratings for generating projective or tomographic phase contrast recordings of a subject are widely known. By way of example, reference is made to the European patent application EP 1 447 046 Al and the German patent applications (not yet published that the priority date of the present application) with the file references 10 2006 017 290.6, 10 2006 015 358.8, 10 2006 017 291.4, 10 2006 015 356.1 and 10 2006 015 355.3.
For imaging by ionizing rays, in particular X-rays, principally two effects can be observed which occur when the radiation passes through matter, namely the absorption and the phase shift of the radiation passing through a subject. It is known that in many cases, the phase shift when a ray passes through a subject depends much more strongly on small differences in the thickness and composition of the penetrated matter than the absorption effects do. Structures of a subject, it particular the soft structures of a patient, can thereby be recognized better.
For such phase contrast radiography or phase contrast tomography, the phase shift due to the object must be evaluated. Here, similarly as conventional absorption contrast X-radiography or absorption contrast X-ray tomography, both projective images of the phase shift can be compiled or even tomographic representations of the phase shift can be calculated from a multiplicity of projective images.
The phase of an X-ray wave cannot be determined directly, rather only by interference with a reference wave. The phase shifts relative to reference waves or neighboring rays can be measured by using interferometric gratings. In respect of interferometric measurement methods, reference is made to the documents cited above. In these methods, coherent X-radiation is passed through a subject, then delivered through a phase grating with a period adapted to the wavelengths of the radiation so as to create an interference pattern, which is phase shifted depends on the phase shift occurring in the object. This interference pattern is measured by a subsequent analysis-detector arrangement, so that the phase shift can be determined with position resolution.
The following should essentially be pointed out in this regard:
The emission of X-ray photons from laboratory X-ray sources as well as by conventional synchrotron radiation sources of the first to third generations is subject to stochastic processes. The emitted X-radiation therefore has no spatial coherence per se. In phase contrast radiography and tomography or any interference experiment, however, the radiation of X-ray sources behaves as coherent radiation when the observation angle at which the source appears to the observer or the object, the grating or the detector, is sufficiently small. The so-called spatial coherence length Lc can be provided as a measure of the spatial or transverse coherence of an extended X-ray source
                              L          c                =                  λ          ⁢                                    a              s                        .                                              (        1        )            
Here, λ is the wavelength, s is the transverse source size and a is the source-observation point distance. Many authors also refer to half the above-defined value as the spatial coherence length. The exact value is incidental; what is important is that the coherence length Lc is large compared to the (transverse) dimension of the spatial region from which rays are intended to interfere with one another.
In the context of the patent application, the term coherent radiation is intended to mean radiation which leads to the formation of an interference pattern under the given geometries and given distances of the X-ray optical gratings. It is self evident that the spatial coherence and therefore the spatial coherence length is always determined by the trio of quantities: wavelength, source size and observation distance. With a view to compact formulation, this fact has been abbreviated to terms such as “coherent X-radiation”, “coherent X-radiation source” or “point source for generating coherent X-radiation”. The basis for these abbreviations is that the wavelength or the energy E of the X-radiation in the applications discussed here is limited by the desired penetratability of the subject on the one hand and the spectrum available in laboratory X-ray sources on the other hand. The distance a between the source and the observation point is also subject to certain restrictions in laboratory equipment for nondestructive material testing or medical diagnosis. This usually leaves only the source size s as a single degree of freedom, even though the relationships between source size and tube power set narrow limits here.
The requirement for a small or point-like radiation source means that the available intensity is also relatively low. In order to increase the intensity, it has therefore also been proposed to use an X-ray source with a relatively large-area focus and to place an X-ray optical absorption grating, a so-called source grating, in the beam path between the focus and the subject. The large-area focus makes it possible to use larger and therefore more powerful X-ray sources. The narrow slits or gaps of the source grating ensure that all the rays, which have to emerge from the same slit, comply with the requisite spatial coherence. The slit width must satisfy the size requirement given by Equation (1) for the transverse source size s. Correct superposition, at least in terms of intensity, of the maxima and minima of the standing wave field is possible between the photons from slit to slit of the source grating with suitable tuning of the source grating period g0 and the interference pattern period g2 as well as the distance 1 between the source grating G0 and the phase grating G1 and the distance d between the phase grating G1 and the interference pattern, according to:g0/g2=1/d.  (2)
In the abbreviated formulation of the patent application, the term “quasi-coherent radiation” or “quasi-coherent radiation source” is used in this context.
The temporal or longitudinal coherence of the radiation is associated with the monochromaticity of the X-radiation or of the X-radiation source. The X-radiation of intense characteristic lines usually has a sufficient monochromaticity or temporal coherence length for the applications discussed here. Upstream monochromators or selection of the resonant energy via the bar height of the phase grating can also filter out a sufficiently narrow spectral range from a Bremsstrahlung spectrum or synchrotron spectrum, and thus satisfy the requirements for the temporal coherence length in the present arrangements.
A problem with these X-ray optical transmission gratings is that the production of such gratings, which require a large aspect ratio (=ratio of the bar height to width of the grating gap), is very elaborate. Furthermore, the precision of the production deteriorates significantly with an increasing aspect ratio.